It is well known that the spatial distribution of microwave energy in the cavity of a microwave oven tends to be non-uniform and that this causes undesirable hot and cold spots in food being cooked. Accordingly, extensive research and development work has been done with the object of improving the time averaged spatial distribution of energy in the food. The most common approach to improving uniformity has been to use a mode stirrer to randomize reflections and thereby move the spatial locations of the maxima and minima interference between modes. Although providing some improvement, mode stirrers have only been marginally effective. Another marginal approach has been to move the food through the complex microwave field so that the hot and cold spots in the food are not stationary.
The prior art has also used a primary radiator or antenna that is normally positioned in the top center of the cavity. From there, it radiates energy downwardly to the food in a pattern characteristic of antenna design. Rotation of the antenna then sweeps the pattern over the food to improve uniformity on a time averaged basis. Further improvement has been obtained by increasing the diversity of field patterns produced and one way of doing that has been to offset the antenna from the center of rotation.
It has also been well known that heating uniformity can be improved by radiating a pattern that is circularly polarized. By this it is meant that orthogonal components are approximately equal with a phase difference of 90.degree.. For example, a wave propagating horizontally would be circularly polarized if its vertically and horizontally polarized components were equal and also separated by a quarter cycle in time or space. A microwave oven cavity excited by circular polarization is equivalent to a cavity with two orthogonal independent sets of modes excited simultaneously. The randomization and thereby smoothing of the maxima and minima interference as compared to a linearly polarized pattern results because the orthogonal components may reflect differently. More specifically, in the case of a side wall reflection from an antenna mounted at the top of the cavity, the direction of rotation of the reflected circularly polarized wave is reversed. Accordingly, its constructive and destructive interference with a wave propagating directly from the antenna are more random because they have a different sense of rotation.
In U.S. Pat. No. 4,336,434, issued June 27, 1982, circular polarization was described as being provided in a microwave oven using a waveguide that propagated circular polarization or using X slot apertures. Although phase shifters were described between the X slot apertures to provide beam steering for improving uniformity, the pattern itself was fed from a stationary location.
A helical or spiral antenna has also been used to provide circular polarization in a microwave oven (U.S. patent application Ser. No. 709,237, filed March 6, 1985). Using a coaxial rod conductor, the microwave energy was coupled from a waveguide to a conductive strip which included a radial strip conductor or microstrip transmission line connected to a helical strip antenna. By rotating the coaxial rod conductor about its axis, the circularly polarized pattern was made to move in a circular path within the cavity. This had the advantage of varying the propagation path of the reflected and direct waves. For example, the angle of arrival and phase of the waves at any spatial point were time varying. A drawback, however, of that helical antenna is that it had a relatively high profile because at least a segment of the spiral had to be cylindrical as compared to flat in order to balance the orthogonal components by gradually sloping away from the ceiling ground plane. Accordingly, either the helical antenna had to be positioned in a ceiling dome, or the usable microwave cavity volume was reduced. Further, when this microwave feed structure was supported on a vertical post directed upwardly from a cross bar of a ceiling suspended cradle, the spacing between the cradle and the ceiling had to be unduly large so that the microwave feed structure could be rotated therebetween.